Contact screens

ABSTRACT

A CONTACT SCREEN HAVING AN EXTENDED TONAL RANGE COMPRISES FIRST AND SECOND ZONES OF MAXIMUM DENSITY, ONE OF THE ZONES BEING A ZONE OF MAXIMUM DENSITY AND THE OTHER OF THE ZONES BEING A ZONE OF MINIMUM DENSITY. THE SCREEN INCLUDES A PATTERN OF CROSSED LINES WHICH CONSTITUTE ONE OF THE ZONES AND WHICH FORM THE BOUNDARIES OF AREAS OF OTHER DENSITIES WHICH INCLUDE THE OTHER OF THE ZONES. EACH AREA INCLUDES A PLURALITY OF DOTS OF DIFFERENT VIGNETTED DENSITY ARRANGED AROUND THE CENTER OF THE AREA. THE REMAINING PART OF EACH AREA HAS A VIGNETTED DENSITY WHICH VARIES FROM A VALUE NEARER TO THAT OF THE CROSSED LINES AT THE CENTER OF THE REMAINING PART OF THE AREA TO A VALUE NEARER TO THAT OF THE OTHER ZONE AT A LOCATION NEARER TO THE LINES FORMING THE BOUNDARIES OF THE AREA. THE CONTACT SCREEN IS OF USE IN THE PHOTOGRAVURE AND SIMILAR PROCESSES AND MAY BE A POSITIVE-PRODUCING SCREEN OR A NEGATIVE-PRODUCING SCREEN.

H. MIDDLEMISS CONTACT SCREENS June 4, 1974 4 Sheets-Sheet 1 Filed March 29 1972 vBl H. vMlDDLEMlss CONTACT SCREENS June 4, 1974 4 Shine ts-Sheet 2 Filed March 29 1972 June 4, 1974 H, MlDDLEMlss 3,814,608

CONTACT *SCREENS Filed March 29 1972 4 Sheets-Shut S if* Q- Il l. I

June 4, 1974 H. MIDDLEMlss 3.814.608

CONTACT SCREENS Filed March 29, 1972 l 4 Shouts-Shut 4 United States Patent O1 ce 3,814,608 Patented June 4, 1974 3,814,608 CONTACT SCREENS Herbert Middlemiss, Nottingham, England, assigner to Howson-Algraphy Limited, Leeds, England Filed Mar. 29, 1972, Ser. No. 239,251 Int. Cl. G03f 5/00 U.S. Cl. 96-116 6 Claims ABSTRACT F THE DISCLOSURE A contact screen having an extended tonal range cornprises first and second zones of limiting density, one of the zones being a zone of maximum density and the other of the zones being a zone of minimum density. The screen includes a pattern of crossed lines which constitute one of the zones and which form the boundaries of areas of other densities which include the other of the zones. Each area includes a plurality of dots of different vignetted density arranged around the center of the area. The remaining part of each area has a vignetted density which varies from a value nearer to that of the crossed lines at the center of the remaining part of the area to a value nearer to that of the other zone at a location nearer to the lines forming the boundaries of the area. The con tact screen is of use in the photogravure and similar processes and may be a positive-producing screen or a negative-producing screen.

This invention relates to contact screens and is concerned with contact screens of the cross-lined vignetted type.

Negative-producing contact screens are known for use in converting a positive image having a variety of continuous tones into a negative image having a variety of crossed lines of varying width. The negative image produced using such conventional screens is used to produce a positive image having square opaque dots of varying size each with their sides parallel one with the other. The positive image is then used to prepare the metal printing plates or cylinders required by the intaglio half tone photogravure process. Such plates or cylinders include cells of varying depth formed below the surface of the metal by etching. The tones reproduced by the cells on printing depend on the size of the cells.

The lines on such conventional negative-producing screens are usually made up from a criss-cross pattern of lines of minimum density forming the boundaries of darker i.e. higher density areas which areas are increasingly vignetted so that they have a central core of maximum density.. Half tone negatives made from such conventional negative-producing screens have a restricted range of tone. This arises through the necessity to retain throughout the tonal range a continuous pattern of crossed lines. Also lines of a minimum width are required so that the walls of the cells of the printing plate subsequently produced from the negative will withstand the preparatory stages in the etching of the metal and also the wear experienced during the actual printing process. As a result of this limitation the long half tone ranges applicable to other printing processes are not generally used in the intaglio photogravure process.

Positive-producing contact screens are also known for use in converting a negative image having a variety of continuous tones into a positive image having a variety of crossed transparent lines of varying width. The positive images these screens produce are formed by square opaque dots of varying size each with their sides parallel one with the other and are also used to prepare the plates and cylinders required by the intaglio half-tone process.

The lines on such conventional positive-producing screens are usually made up from a criss-cross pattern of lines of maximum density forming areas within the lines which are vignetted with a decreasing density so that they have a central core of minimum density. Positives prepared from such positive-producing screens have similarly had a restricted range of tone because of the necessity of retaining throughout the tonal range a continuous pattern of crossed transparent lines and, again, lines of minimum width are necessary. As in the case of the negative-producing screens therefore, the long half-tone ranges are not realised.

It has been found that it is possible to provide a contact screen from which there can be produced half-tone negatives or positives, as the case may be, having an extended tonal range for use in the photogravure process or, indeed, in any process which requires a continuously linked half-tone structure by devising the contact screen so that it will result in a structure of larger area than normal which progressively divides into smalle1 cells which in turn become progressively eliminated. Thus, the darker areas of the resultant printed material are darker than normal by reason of the larger cell capacity and the lighter areas are lighter than normal by reason of the removal of cells.

According to the present invention there is provided a contact screen inclding first and second zones of limiting density, one of said first and second zones being a zone of maximum density and the other of said first and second zones being a zone of minimum density, said screen having a pattern of crossed lines which constitute one of said first and second zones and which form the boundaries of areas having other densities wherein each of said areas includes dots of different vignetted density arranged around the centre of the area and wherein the remaining part of each of said areas has a vignetted density which varies from a value nearer that of said one of said first and second zones at the centre of the area to a value nearer that of said other of the first and second zones at a location nearer the lines forming the boundaries of the area.

The contact screen may be a negative-producing screen. In this case, the zone constituted by the crossed lines is the zone of minimum density, and the areas bounded by the lines are areas of higher density. Each of said areas of higher density includes dots of different vignetted density arranged around the centre of the area and the remaining part of each of said areas of higher density has a vignetted density which increases from the centre towards the lines forming the boundaries of the area.

In one embodiment of a negative-producing screen, the pattern comprises transparent crossed lines which form the boundaries to square darker areas lying Within each adjacent pair of crossed lines. Four square dots of higher density are located in each square, one in each corner, with their sides parallel with the lines. The four dots have the same size but each dot has a different vignetted density, the density increasing towards the centre of each dot. The two darkest dots are diagonally opposed and the two lightest dots are disposed on the oppoiste diagonal. The remaining space in the square is in the form of a cross having a width equal to that of the crossed lines and having an increasing vignetted density from its center towards the lines defining the square.

In a convenient arrangement, the pattern comprises of the transparent crossed lines to the inch and 200 lines of the square dots to the inch.

The embodiment above described forms a photogravure half-tone negative image which progressively follows the configuration and densities of the screen pattern. Therefore, when photosensitive material is exposed using such a screen, the minimum light action produces only a pattern of opaque crossed lines corresponding to the crossed transparent lines of the screen. As the light action is increased, diamond shaped dots corresponding to the center of each of the square darker areas of the screen appear centrally within each of the squares defined by the crossed opaque lines and these are followed by similar dots placed on and midway between the crossed opaque lines in both the horizontal and vertical directions. With further increase in light action these dots enlarge and join so that the original opaque crossed-lined formation is divided into four transparent square dots of equal size and corresponding to the transparent crossed lines of the screen and the vignetted cross within each of the square darker areas of the screen. As the light action is increased further these dots diminish in size equally until a predetermined point is reached at which they diminish in size unequally and finally disappear in an order which is governed by the densities of the dots in each of the square darker areas of the screen.

As a result of these progressive changes at each end of the tonal range an increase in the range is obtained which can be fully exploited or else used to produce a greater tonal latitude than would otherwise be available.

If desired, the contact screen may be a positive-produc ing screen. In this case, the zone constituted by the crossed linesis the zone of maximum density and the areas bounded by the lines are areas of lower density. Each of said areas of lower density includes dots of different vignetted density arranged around the centre of the areas and the remaining part of each of said areas of lower density has a vignetted density which decreases from the centre towards the lines forming the boundaries of the area.

In one embodiment of a positive-producing screen, the pattern comprises relatively opaque crossed lines which form the boundaries to square lighter areas lying within each adjacent pair of crossed lines. Four square dots of lighter density are located in each square, one in each corner, with their sides parallel with the lines. The four dots have the same size but each dot has a different vignetted density, the density decreasing towards the centre of each dot. The two darkest dots are diagonally opposed and the two lightest dots are disposed on the opposite diagonal. The remaining space in the square is in the form of a cross having a width equal to that of the crossed lines and having a decreasing vignetted density from its centre towards the lines defining the square.

In a convenient arrangement, the pattern comprises 100 of the opaque crossed lines to the inch and 200 lines of the square dots to the inch.

The embodiment above described forms a photogravure half-tone positive image by progressively transmitting light on to a photosensitive material in contact with the screen. When photosensitive material is exposed using such a screen, the maximum light action produces only a pattern of transparent crossed lines corresponding to the crossed opaque lines of the screen. As the light action is decreased, diamond shaped dots corresponding to the centre of each of the square lighter areas of the screen appear centrally within each of the squares defined by the crossed transparent lines and these are followed by similar dots placed on and midway between the crossed transparent lines in both the horizontal and vertical directions. With further decrease in light action these dots enlarge and join so that the original transparent crossed-lined formation is divided into four opaque square dots of equal size and corresponding to the opaque crossed lines of the screen and the vignetted cross within each of the square lighter areas of the screen. As the light action is decreased further these dots diminish in size equally until a predetermined point is reached at which they diminish in size unequally and finally disappear in an order which is governed by the densities of the dots in each of the square lighter areas of the screen.

As a result of these progressive changes at each end of the tonal range an increase in the range is obtained which can be fully exploited or else used to produce a greater tonal latitude than would otherwise be available.

The screens of the present invention may be made in any required color. The screens can be produced in conventional manner by projecting an appropriate pattern of light through the apertures of a crossed-line screen onto photosensitive material and developing the thus exposed material.

For a better understanding of the invention and to show how the same may be carried out, reference will now be made by way of example, to the accompanying drawing, in which:

FIG. l shows a portion of a negative-producing contact screen of the invention considerably magnified,

FIG. 2 shows representations of a screen negative image obtained using the screen of FIG. 1 and shown at a lesser magnification,

FIG. 3 shows in diagrammatic form the projection of a single image of a light pattern appropriate to the production of the screen shown in FIG. l,

FIG. 4 corresponds to FIG. 3 but shows the projection of four images of the light pattern,

FIG. 5 shows a portion of a positive-producing contact screen of the invention considerably magnified,

IFIG. 6 shows representations of a screen positive image obtained using the screen of FIG. 5 and shown at a lesser magnification,

FIG. 7 shows in diagrammatic form the projection of a single image of a light pattern appropriate to the production of the screen of FIG. 5, and

FIG. 8 corresponds to FIG. 7 and shows the projection of four images of the light pattern.

For greater ease of recognition, corresponding parts of the screens, the half-tone images, the patterns, and the images produced by the patterns are denoted by the same references and the images produced by the patterns are shown as developed images.

Referring to FIG. l, the screen includes a pattern of transparent crossed lines and these lines are bracketed together and indicated by reference A in both horizontal and vertical directions. These lines constitute that zone of the screen which is of minimum density and form the boundaries to square darker areas of which only one is completely shown. The square darker areas include that zone of the screen which is of maximum density. In each of the square darker areas there are located for square dots each having the same size. One dot is located in each corner of each area with its sides parallel with the transparent crossed lines. The four dots located in the completely shown square are denoted by references B, B11, B2 and B3. Each dot is vignetted and each has a different density, dot B being the darkest and dot B3 being the lightest. The density of each dot increases towards the centre of the dot. The two darkest dots =B and B1 are diagonally opposed as are the two lightest dots B2. and B3. The space remaining in the square is in the form of a cross which has a line width which is similar to that of the transparent crossed lines and this has a vignetted density increasing from its centre indicated at A1 towards the transparent crossed lines. The pattern of transparent crossed lines has lines to the inch, these lines being indicated in the vertical direction only at C and the pattern includes 200 lines of dots to the inch, these lines of dots being similarly indicated at C1.

Reference will now be made to FIG. 2. When the screen is used in conjunction with a continuous tone positive image, or a positive image formed optically, and a high contrast photosensitive material, the light variations from the positive image result in the formation of a sharply defined half-tone photogravure negative image on the photosensitive material which progressively follows the configuration and densities in the screen pattern. Thus the minimum light action, transmitted through the transparent crossed lines only, produces only the crossed opaque lines indicated at A in FIG. 2. Further light action penetrates the slightly greater densities at A1 in FIG. 1 and a corresponding diamond-shaped dot forms centrally within the crossed lines as indicated at A1 in FIG. 2.. The

next step is formed by the peneration of the still greater densities which lie midway between the lines horizontally and vertically as indicated by the four references A2 in FIG. l and the result is four additional dots at A2 in FIG. 2. It will be observed that as the additions occur a widening of the original crossed opaque lines also takes place. At the next step in the progression a complete penetration of the minor screen densities carries the half-tone into the region occupied by the screen dots in the square darker aereas and the'image at this point can be described as transparent square dots of equal size and these are indicated at B, B1, B2 and B3 in IFIG. 2. These transparent dots diminish in size equally, ultimately reaching a point where they reduce in size unequally as the light penetrates in varying proportions the varying densities within the screen dots. Eventually, selected dots disappear in the order of increasing screen dot densities. Consequently, the screen dot of lightest density B3 is the iirst to be penetrated with the corresponding removal of the dot B3 in FIG. 2. Subsequently dots B2 and B1 are eliminated in similar fashion.

As a result of the screen pattern used a tonal range is achieved which is longer than that obtained from a screen having a conventional pattern since the darker areas of the printed image are obtained from cells having a greater capacity and the lighter areas are obtained as a result of the progressive removal of cells. Thus, the two effects combined produce a high degree of tonal range extension while still resulting in a printing plate having a continuously joined cell structure as required by the intaglio halftone photogravure process.

The contact screen of FIG. 1 is produced by projecting an appropriate pattern of light through the apertures of a cross-lined screen onto a photosensitive material and developing the exposed material. The appropriate pattern of light is achieved by shining a light through a pattern of varying densities corresponding to the pattern of light required. A single image of this light pattern is thus projected through each aperture of the cross-lined screen thus forming multiple images which combine to produce the pattern of vcrossed lines and dots. The light pattern appropriate to the screen of FIG. 1 is produced using the pattern denoted by reference E in FIG. 3. The pattern comprises an opaque border E1 surrounding a square area in the four corners of which are placed smaller square areas of lighter density and provided with extended diagonals one of which is denoted by reference E2. The square areas are denoted by references B, B1, B2 and B3. Each of the four square areas is vignetted with a decreasing density to its center and each has a different density with the two of lightest density (B and B1) diagonally opposed and the two of darkest density (B2 and B3) on the opposite diagonal. The space remaining forms a cross and this has a decreasing vignetted density from its centre A1. In use, light from a source D is shone through the pattern E to form a light pattern which is projected through a cross-lined screen F to form images of the light pattern which impinge on a photosensitive material indicated at G. Each aperture of the crossed-line screen F produces a single image on the material G (FIG. 3) and hence a plurality of images is produced on the material G because of the plurality of apertures of the screen F (FIG. 4). It will be appreciated that the continuous crossed transparent lines of the screen pattern as shown and indicated at A in FIG. 4 are the result of the projection of multiple square images of such a size as will form crossed lines of similar width as the lines at A1, by reason of the space existing between each image of the light pattern produced by the screen F. The function of the extended diagonals as indicated at EZ is to sharpen the corners of dots which would otherwise tend to be rounded oi.

Referring to FIG. 5, the screen includes a pattern of relatively opaque crossed lines and these lines are bracketed together and indicated by reference H in both horizontal and vertical directions. These lines constitute that zone of the screen which is of maximum density and form the boundaries to square lighter areas ofv which only one is completely shown. The square lighter areas include that zone of the screen which is of minimu-m density. In each of the square lighter areas there are located four square dots of still lighter density, each dot having the same size. One dot is located in each corner of each area with its sides parallel with the relatively opaque crossed lines. The four dots located in the completely shown square are denoted by reference J, J 1, J2 and J3. Each dot is vignetted with a decreasing density to its center and each has a dilerent density, dot J being the lightest and dot J3 being the darkest. The two lightest dots J and J1 are diagonally opposed as are the two darkest dots J2 and J3. The space remaining in the square is in the form of a cross which is similar in line width to that of the relatively opaque crossed lines and this has a vignetted density which decreases from its center indicated at H1 towards the lines. The pattern of opaque crossed lines has lines to the inch, these lines being indicated in the vertical direction only at K and the pattern includes 200 lines of dots to the inch, these lines of dots being similarly indicated at Kl.

Reference will now be made to FIG. 6. When the screen is used in conjunction with a continuous tone negative image formed optically or otherwise and a high contrast photosensitive material, the light variations obtained from the negative image result in the formation of a sharply defined half-tone photogravure image on the photosensitive material which progressively follows the configuration and densities in the screen pattern. Thus the maximum light action which pentrates all the densities with the exception of the opaque crossed lines produces only the crossed transparent lines indicated at H in FIG. 6. A lesser light action does not penetrate the slightly lesser densities at H1 in FIG. 5 and a corresponding transparent diamond-shaped dot forms centrally within the crossed lines as indicated at H1 in FIG. 6. The next step is formed by the resistance to diminishing` light action of the still lighter densities which lie midway between the lines horizontally and vertically as indicated by the four references H2 in FIG. 5 and the result is four additional dots at H2 in FIG. 6. It will be observed that as the additions occur a widening of the original crossed lines also takes place. At the next step in the progression a complete resistance to light by the major screen densities carries the half-tone into the region occupied by the screen dots in the square lighter areas and the image at this point can be described as opaque square dots of equal size and these are indicated at J, J1, J2 and J3 in FIG. 6. These dots diminish in size equally, ultimately reaching a point where they reduce in size unequally as the light is resisted in varying proportions by the varying densities within the screen dots. Eventually, selected dots disappear in the order of their decreasing screen dot densities. Consequently the screen dot of highest density J 3 is the first to resist the light with the corresponding removal of the dot I3 in FIG. 6. Subsequently dots J2 and J1 are eliminated in similar fashion.

As a result of the screen pattern used a tonal range is achieved which is longer than that obtained from a screen having a conventional pattern since the darker areas of the printed image are obtained from cells having a greater capacity than conventionally is the case. The lighter areas are reproduced lighter as a result of the progressive removal of cells. Thus, the two affects combined produce a high degree of tonal range extension while still resulting in a printing plate having a continuously joined cell structure as required by the intaglio half-tone photogravure process.

The positive-producing contact screen of FIG. 5 is produced by an analogous method to that described with reference to FIGS. 3 and 4 for the production of the negative screen of FIG. l. In this case, however, the light pattern used is produced using the pattern denoted by reference M in FIG. 7. The pattern comprises a transparent square border indicated by H surrounded by opacity indicated by M1. The transparent square border forms the boundary to the square darker area within and in the four corners of which are placed smaller square areas of darker density which are provided with extended diagonals. These areas are denoted by references J, J 1, J2 and J3 and one of the extended diagonals is denoted by reference M2. Each of these four areas is vignetted with an increasing density to its center and each has a diierent density with the two darkest J and J 1 diagonally opposed and the two lightest J2 and J3 on the opposite diagonal. The space remaining forms a cross and this has an increasing vignetted density from its center indicated at H1.

In use, light from a sourceL is shone through the pattern M to form a light pattern which is projected through a crossed-lined screen N to form images of the light pattern which impinge on photosensitive material P. Each aperture of the crossed line screen N produces an image on the material P (FIG. 7) and hence a plurality of images is produced on the material P because of the plurality of apertures of the screen F (FIG. 8). It will be appreciated that the continuous crossed opaque lines of the screen pattern as shown and indicated at H in FIG. 8, are the consequence of the multiple linkage of the opaque border around each single image, as indicated at H on P shown in FIG. 7 and this is formed by the transparent border indicated at H on M of FIG. 7.

I claim:

1. A negative-producing contact screen having a pattern of crossed lines which constitute a zone of uniform minimum density and which form the boundaries of areas of greater density wherein each of said areas of greater density includes a plurality of spaced vignetted dots arranged around the center of the area, the vignetted density of each dot diering from the vignetted density of each of the other dots in the same said area, the dots of each area being of the same size and having different vignetted densities which increase toward the dot centers, and wherein the remaining part of each of said areas lying between said spaced dots has a vignetted density which increases from the center of the area toward the lines forming the boundaries of the area.

2. A contact screen as claimed in claim 1, wherein each of said areas is a square area.

3. A contact screen as claimed in claim 2, wherein:

(i) each of said areas includes one of said dots in each of its four corners,

(ii) the four dots of each of said areas are square in shape and have their sides parallel to the crossed lines so that said remaining part of each of said areas is in the form of a cross having a width equal to that of the lines,

(iii) the four dots of each of said areas are of the same size and have different vignetted densities which increase towards the dot center, and

(iv) in each of said areas, the two dots of higher density are located at opposite ends of one diagonal of the area and the two dots of lower densityr are located at opposite ends of the other diagonal of the area.

4. A positive-producing contact screen having a pattern of crossed lines which constitute a zone of uniform maximum density and which form the boundaries of areas of lesser density wherein each of said areas of lesser density includes a plurality of spaced vignetted dots arranged around the center of the area, the vignetted density of each dot differing from the vignetted density of each of the other dots in the same said area, the dots of each area being of the same size and having different vignetted densities which decrease toward the dot centers, and wherein the remaining part of each of said areas lying between said spaced dots have a vignetted density which decreases from the center of the area toward the lines forming the boundaries of the area.

5. A contact screen as claimed in claim 4, wherein each of said areas is a square area.

6. A contact screen as claimed in claim 5, wherein:

(i) each of said areas includes one of said dots in each of its four corners,

(ii) the four dots of each of said areas are square in shape and have their sides parallel to the crossed lines so that said remaining part of each of said areas is in the form of a cross having a width equal to that of the lines,

(iii) the four dots of each of said areas are of the same size and have different vignetted densities which decrease towards the dot center, and

(iv) in each of said areas, the two dots of lower density are located at opposite ends of one diagonal of the area and the two dots of higher density are located at opposite ends of the other diagonal of the area.

References Cited UNITED STATES PATENTS 3,493,381 2/1970 Maurer 96-116 3,565,527 2/1971 Crespi 96-1 16 3,164,470 l/1965 Rebner 96-116 3,122,436 2/1964 Watter 96-116 3,132,946 5/1964 Scott 96-116 RONALD H. SMIT-H, Primary Examiner H. iL. GOODROW, Assistant Examiner U.S. Cl. X.R. 96-45 

